The communications industry has been looking eagerly toward providing broadband, two-way communications to business and homes alike. Potential applications include computer networking, telephony, interactive multi-media and entertainment such as video-on-demand. Accordingly, the industry has been viewing fiber-to-the-home (FTTH) technologies as an attractive way to provide pervasive broadband communications. Advantageously, once optical fiber is installed to a user's home or business, the bandwidth can be upgraded as demand warrants without requiring the installation of new cabling and the attendant cost and disruption.
While many FTTH approaches are possible, one particularly attractive approach is described in United States Provisional Patent Application Serial No. 60/075,362 filed on Feb. 20, 1998 by the present inventors for the present assignee and incorporated herein by reference. With this FTTH approach, communications signals directed to users (downstream) are transmitted through a passive optical network (PON) and are split for distribution to a number of users as appropriate. Conversely, signals that emanate from the users are combined as they proceed upstream through the PON.
Unfortunately, however, passive optical networks that utilize combining signals may suffer from a degradation known in the art as optical-beat interference (OBI) whenever two or more output sources (lasers) operate simultaneously. The RF spectrum of OBI produced by multiple lasers at a common photodetector is substantially proportional to the convolution of the electric-field spectral densities of each operating laser. Importantly, if portions of the spectra are closely spaced in optical frequency, OBI may fall in a desired signal band.
Consequently, a need exists for methods and apparatus that reduces OBI in optical communications systems.